Geologic Time Scale

The geologic time scale is a guide to the major eras and periods of the earth's history, from the very beginning of the planet to now. Only a tiny fraction of all of this time have humans been keeping written history. Scientists depict geologic time with the oldest at the bottom, just as it would appear in the rocks they uncover. The oldest rocks and fossils are laid down first. As newer rocks and sediments are added, they accumulate on top of the older rocks, layer by layer. The oldest fossils would be deeper and the youngest fossils closer to the surface. For instance, if you were walking down into the Grand Canyon, you would start with the most recent rock formations, and the deeper you went down in, the older the rocks would be along the canyon walls. In this table of geologic time, the first organisms to appear on earth will be at the bottom of the figure, and the most recent at the top.

Information compiled from Dixon, D., et al. 1988. The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. Macmillan Publishing Company, NY. NY, and the Geologic Time Scale, by the Department of Paleobiology, National Museum of Natural History: http://www.nmnh.si.edu/paleo/dino/timescal.htm

ERA	PERIOD	MYA*	LIFE AT THE TIME
Cenozoic Era Age of Recent Life or The Age of Mammals	Quaternary	0.01 - 5	Many mammals vanish during a vast ice age. Modern humans emerge and spread world wide. Species of plants and animals are similar to what we see today.
	Tertiary	5 - 145	The continents have moved to positions near where they are today. Flowering plants thrive and diversify. Vast forests exist in the tropical and temperate environments. Mammals spread and diversify.
Mesozoic Era Age of Medieval Life (Time of the Ruling Reptiles)	Cretaceous	145 - 200	North and South America begin to split apart. India is a separate continent, In the north are Euramerica and Asiamerica, with differing plant and animal species. Dinosaurs diversify and rule until the end of the Cretaceous period when they die out in a mass extinction along with many other marine and terrestrial species. Flowering plants appear, and insects begin to pollinate.
	Jurassic	200- 250	The supercontinent, Pangea, splits and the Atlantic Ocean appears separating Asia from the Americas and Africa. The age of the Ruling Reptiles is in full swing. Dinosaurs rule the earth and pterosaurs rule the skies. Dinosaurs are much larger and include giant herbivores. The first birds appear.
	Triassic	250 - 295	The first dinosaurs and mammals appear; they tend to be small and quick predators who run on their back legs. Cycads abound, seed ferns go extinct. Ammonites are common. Dinosaurs, crocodiles and pterosaurs emerge and diversify.
Paleozoic Era Age of Ancient Life	Permian	295 - 362	All land is in one giant continent, Pangea. Ferns, seed ferns and conifers abound. Insects diversify and spread. Many marine animals, including the trilobites, go extinct.
	Carboniferous	362 - 418	All land is in two great continents, in the north is Euramerica and Gondwanaland is in the south. The first reptiles appear on land, as do the first conifers and cycads. Giant ferns, horsetails, and club mosses are common. Trilobites become less common, graptolites go extinct.
	Devonian	418 - 439	Amphibians and insects begin to invade the land. The earliest ferns and plants with seeds appear. Fish abound.
	Silurian	439 - 490	The first life emerges on land, as plants and invertebrates. Fish develop and split into the bony fishes (teleosts) and the cartilaginous fishes (sharks and rays). Marine invertebrates continue to thrive.
	Ordovician	490 - 543	Corals, bryozoa and graptolites thrive along with the marine life of the Cambrian. The first true vertebrate fish discovered.
	Cambrian	543 - 2500	Marine life abounds, including red and green algae, brachiopods, gastropods, trilobites, sponges. The earliest fishes appear.
Precambrian Era (Time before Life)		2500 – 4600	Time from the birth of the planet approximately 4.6 billion years ago, until the first simple life forms appeared about 3.6 bya, including early bacteria and blue-green algae. The first multicellular animals, such as worms and jellyfish, appeared near the end of this era.